This finally happened again. I tried hitting the 'F' key and it did nothing. I went to windows and selected normal, which put the view back into a normal mode, at which point the 'F' key started working, properly. Weird.
Sounds like you have the "window-ized" application window sized so it's just a tad under the same size as full screen and positioned slightly off-centre. If you accidently then toggle into "windowed" mode you get the phenomenon you describe. Make sure you've gone to full-screen by clicking the center icon in the upper-right corner rather than by clicking and dragging the borders to resize the window.